There is intense interest in the pulp and paper art for the reduction of the amount of chlorine used in bleaching pulp. Chlorine has been shown in many situations to generate poisonous compounds, such as dioxins, in the bleach plant effluent.
A number of steps have been taken to try and minimize the amount of chlorine utilized for bleaching. Some approaches utilize oxygen pre-treatment, and high chlorine dioxide use. Such procedures are not entirely effective, however, because there are limitations in the final brightness that can be achieved, and because chlorine dioxide is a much more expensive bleaching chemical and significantly increases bleaching chemical and significantly increases bleaching costs. To overcome the final brightness limitations, peroxide is sometimes used. However peroxide is also very expensive. A typical old sequence, and a new sequence in which chlorine use is minimized, are C.sub.E EDED, and OC.sub.D E.sub.o DEDP, respectively, wherein O is oxygen treatment, D chlorine dioxide, C chlorine, P peroxide, and E caustic extraction.
Another way in which chlorine usage can be minimized is to use more oxygen--for example a stronger E.sub.O stage. Unfortunately this causes strength (viscosity) losses. However, according to the present invention it has been found that the viscosity loss caused by utilizing more oxygen can be overcome by providing a series of oxygen stages with washing between the stages. It has also been found that while chlorine can be used as the first stage to affect acid removal of metals, pre-treatment of the pulp with a chelating agent, such as DTPA, and/or adding another chelating agent, such as EDTA, and the oxygen reactor, also allows one to achieve better bleaching (a lower Kappa number) without undue loss of viscosity or yield. The lower pH's caused by the oxygen stage combined with the chelating removes metals, which allows operation of the process to lower Kappa numbers.
According to the present invention it is possible to minimize or eliminate chlorine usage in bleaching by utilizing a two (or more) stage oxygen treatment process with washing between the stages and with the first stage operated to control pH between the stages, and with the chelating agent utilized for pre-treatment and/or added to the counter-flow of wash liquid in the wash between the oxygen stages. The gains in brightness, without subsequent viscosity loss, by utilizing the between stage washing are dramatic, being substantially equal to such gains as can be obtained utilizing pre-treatment with DTPA.
According to one aspect of the present invention there is provided a method of bleaching paper pulp comprising the steps of: (a) Effecting oxygen bleaching of the pulp in at least two consecutive stages; and (b) Effecting washing of the pulp between each of said at least two consecutive stages to maximize viscosity for a given degree of bleaching. It is also desirable to practice the step (c), before step (a), of pre-treating the pulp with a chelating agent such as DTPA, and to provide the further step (d), during the practice of step (b), of simultaneously treating the pulp with a chelating agent such as EDTA. For example the EDTA may be added to a countercurrent flow of wash liquid to the pulp in the between stage washing. Exactly two oxygen stages may be utilized, or a number of different oxygen stages. Step (a) is practiced to control the pH between stages so that it is at an acidic level conducive to effective EDTA chelating.
According to another aspect of the present invention a high viscosity bleached paper pulp is produced by practicing the steps of: (a) Effecting oxygen bleaching of the pulp in at least two consecutive stages; and (b) Effecting washing of the pulp between each of said at least two consecutive stages to maximize viscosity for a given degree of bleaching.
The invention also contemplates a method of delignifying a suspension of comminuted cellulosic fibrous material (pulp) at a consistency of about 6-15% comprising the steps of sequentially and continuously: (a) Subjecting the suspensions, while at a consistency of about 6-15%, to a first oxygen delignification treatment. (b) Washing the suspension, while at a consistency of about 6-15%. And, (c) subjecting the suspension, while at a consistency of about 6-15%, to a second oxygen delignification treatment. Step (b) is practiced by a countercurrent flow of wash liquid, and during the practice of step (b) a chelating agent is preferably added to the wash liquid. Prior to step (a) there preferably is also a step of pre-treating the suspension with a chelating agent.
It is the primary object of the present invention to provide a method for bleaching paper pulp or the like in which the amount of chlorine usage is minimized, or chlorine is eliminated entirely, while the viscosity of the paper pulp is maximized and an adequate degree of bleaching (low Kappa number) is achieved.